Color television camera



June 9, 1953 J. w. wENTwoRTH 2,641,643

COLOR TELEVISION CAMERA Filed Dec. 1. 195o EZUE PED

Geli/v l azi/le n lNVENlIOR V J n Yummugw @-2 kzf/ ATTORNEY PatentedJune 9, 1953 COLOR TELEVISION CAMERA John W. Wentworth, Haddoneld, N.J., assignor to Radio Corporation of America, a corporation of DelawareApplication December 1, 1950, Serial No.` 198,532

1 Claim.

This invention relates to improvements in color television cameraswhereby a high degree oi registration is not required and wherein thevarious functions are performed with a minimum number of pick-up tubes.

A system of color television has been developed in which the transmittedsignal consists of a high-definition brightness signal (directlycomparable to an ordinary blaclc-and-white television signal) with alow-delinition chromaticity signal superimposed on it in the form ofmodulation of a frequency-interlaced sub-carrier. The brightness signalrepresents the sum of the brightnesses of the green, red, and bluecomponents of the televised scene. The chromaticity signal is obtainedby applying simultaneous signals representing the intensities of thegreen, red, and blue components of the scene to a threephase modulatorconnected in such a way that the output signal of the modulator has anamplitude corresponding to saturation and an instantaneous phasecorresponding to hue. The chromaticity signal may have a much narrowerband width than the brightness signal because the human eye does not seedetail in color as well as it sees detail in black-and-white.

In the prior art, color television cameras are disclosed whereinseparate pick-up tubes are employed to derive the signals correspondingto the intensity of each of the component colors. In other systems, asingle camera is employed to pick-up the brightness or black-and-whitesignal and two additional cameras are employed to derive the essentialcolor information. However,

when the latter type of camera is employed in a white information, inorder to derive a signal that is representative of the third color. Ineach of these systems, three pick-up tubes are there-D fore required.

In still other systems, cameras are disclosed in which an attempt ismade to use a single pick-up tube and to obtain color separation bymeans of line or dot color filters either external'or built into thetube itself. In such systems, adequate 'resolution is'obtained only byusing a very large Vgreatly relaxed, and a strip type of filter withrelatively few lines can be yused for color separation without requiringextreme linearity and stability in the scanning circuits.

It is another object of this invention to provide an improved colortelevision camera wherein the number of pick-up tubes employed isreduced and the normal stringent requirements for exact registration aregreatly relaxed.

In accordance with this invention, however, the problem of registrationis greatly minimized as the black-and-white information is derived byone pick-up'tube and the color information is The way in which the aboveobjectives may beA obtained will be better understood from a detailedconsideration of the drawings, in which:

Figure l illustrates in block diagram form a color television cameraincorporating the features of this invention, .and y Figure 2 'is agreatly enlarged View of a small section of a type of filter that may beemployed in the color pickmp tube of Figure l, as well as certainwaveforms useful in the explanation of the operation of the invention.

Turning now to Figure l, -there is shown means for optically focusing animage to be televised onto the photo cathodes of two pick-up tubes 2 andt respectively. shown, of an objective lens 5 and a hali-silve'redmirror 8, As is well known to `those skilled in the art, it is customaryto place the pick-up tubes 2 and 4, so that their principal axes areperpendicular, the principal axis of one of them coinciding with theprincipal axis of the objective lens 6. The half-silvered mirror 8 ismounted at a angle with reference to each of the principal axes asshown.

The pick-up tube 2 operates in a conventional manner to supplybrightness signals, such as are used in present monochrome systems, toan amplifier 9. The pick-up tube 2 is ofcourse equipped with means forprojecting an electronic beam onto the photo cathode and deflectioncoils H) for causing the beam toscan a raster on the target. Forpurposes of illustration, it will be assumed that the camera is tooperate within It may be comprised,l

' in the line lter.

tube is provided with an electron gun or other` means for projecting abearnof electrons toward a target and deflection coils IIy adaptedtdcause the electron beam to scan a raster on the target.

Y Between the photo cathodeof. the pick-up tube A.

and the half-silvered mirror 8, there is placed a series of linefilters, such as generally indicated by the numeral I2 in Figure 2.Assuming that this; isa three-color systemJ the rst strip in a sequencemay transmit only bluelight, a second strip only green light, and athird strip only red light. In between eachl of these-sequences, thereis positioned a.v clear filter that is a neutral filter capable oftransmitting all the various colors of light, The filter I2 ispreferably an integral part of the tube in order that'it may be mountedas close'to the photo cathode of the pick-up tube liaspossible.

Thelter I2 is positioned so 4that these line stripsare substantiallyperpendicular to the direction followed by the beam as it scans itsraster. Thus the signals derived by the beam and present at the-outputcircuit ofr theV pick-up tube il successively represent the intensitiesof the-light transmitted by the blue, green, red, and clear line orstrip filters.

The actual number of strips in the linefilter I2 is determined by theresolution desired from the color channels, The numberof pictureelements of each color across anyline can be no greatery than the numberof strips of that color This number may be as low as iifty(corresponding to a resolution of 0.5 mc.) to give commerciallyacceptable pictures, while the upper limit depends on the accuracy withwhich strip filters can be made and the stability of associated circuits(filters of 100 or more lines of each color to the inch should bepractical at the present state of the art).

A waveform I4 of Figure 2 illustrates the type of signal Ythat maybeproduced at the output of the color tube 4, the dotted line I5representing the black level. in suchv a way with respect to the filterI2 that those portions of it appearing in vertical registry with thestrips of filter I2 correspond to the amplitude of the signal generatedas theV beam in the pick-up tube d crosses that portion of its targetupon which the image passing through the corresponding filter stripfalls.

In view of the fact that all the component colors may pass with but verylittle reduction through the clearY strips of light filter I2, theamplitude of the waveform Ill produced in response to this transmittedlight is generally greater than the amplitude of signal produced inresponse to the different component colors of light that pass throughthe green, red, and blue strips respectively,

After being amplified inan amplier I6, the signals, such asrepresentedbythe waveform I4, are applied'to two'separateputput leads Il and I9.The output lead IIis connected to anyfcon- The waveform Iii is drawn 4ventional clamp circuit 2U adapted to clamp the Waveform I4 to the peaksof the signals generated in response to the light transmitted by theclear strips as indicated in the waveform 2l. A clamp circuit suitablefor this purpose is illustrated in section 3-11 of Waveforms by Chanceand others (McGraw-Hill, 1949). The output of the clamp circuit 20 isthen applied to an amplitude selector 22 of conventional design, such asfound in sec. 9-3 of Waveforms (op. cit). Those portions of the clampedwaveform 2| that eXceedthe clipping level indicatedby the dotted line-23am-applied in conventional manner so as toi control the, frequency ofan oscillator 24.

During aparticular sequence of signals including a signal from each ofthe color strips and also from aclear strip, it is sometimes possiblethat the amplitude of the signals derived from the information passed bythe clear strips may be less than one or more of the signals passed bythe color strips. This, isa generally unusualsituation and, in order toprevent` the frequency of the oscillatorZsfrombeing undulyaffectediunder such conditions, the time constant of the fre.- quencycontrol circuit can b.e ,made.10ng enough, so as to-include informationfrom a plurality of the clear strip-s. In this way any errors introducedwhen the incise or other unusual effects causes the signals generatedinresponse to the light .passed by the clear strips to have awminimumamplitude in any givensequence may be averaged out.

The output of the. Qscillator 2,4, is applied to any conventionalA pulseforming circuit, such-` as may be found insee. 9-3 ofWaveforms (op.cit). The pulsesshould be relatively narrow withv respect to thedurationA ofthe signals produced in response toanyone ofthe, colorstrips. These pulses, as will be explained hereinafter, are to operategating.V circuits so as toiseparate the signals correspondingtovtheliglit.v intensity passed by the differently colored line filtersin the lil-A ter I2.

The signals, such as illustrated by the Waveform I l in Figure 2 arealsoA applied, over the output lead I9, to aplurality of gating devices26 21, and.28, after being suitably amplied amplifiers 29, 30, and 3|.The pulses derivedjfrom the pulse forming circuit 25 areapplied:directly to the gate 26.- After being delayed by a delay` line 3,2,

l the gate 28, when the signals generatedby the pick-up tube 4correspond to th-e curve I4 in Figure-2, are indicated by the numeral36. In order that a smooth output bederived fromv these pulses, they arepassed through the low-pass filter 39, which provides a smooth outputsuch as indicated by the dotted line 40 in Figure 2. In accordance withwell establishedtheory, the curve 40 exactly represents the average ofthe pulses 3.6 if the low-pass filter hasa cut-off frequency that isone-half the frequency of the pulses 36.

The fact that nosamp-ling control pulses,` appear while thescanningbeamin the pick-up tube @fis scanning a black area. in. the picture:introducesl no serious problem since during this time great accuracy insampling is not necessary.

The overall operation of this particular ernbodiment of the invention isas follows. When the beam in the pick-up tube 4 generates a signal inresponse to light pas-sed by the clear sections of the filter l2, apulse is applied to control the frequency of the locked-in oscillator24, so that it completes one cycle while the beam in the pickup tube 4is scanning one complete sequence of line lters, that is blue, green,red, and clear line filters. The phase of oscillator 24 is so adjustedthat when the pulse generated by the pulse former 25 is applied to thegate 2t without any delay, it opens the gate 26 while the signalsgenerated by the scanning beam in the pick-up tube 4 are representativeof the intensity of the light passed by the blue filter. After passingthrough a suitable delay line 32, the pulse formed by the pulse former25 is applied to the gate 2T. Thus, the gate 2'! is open when thesignals generated by the beam in the pick-up tube 4 correspond to theintensity of the green light passed by the green line filters.Similarly, a further delay line 33 operates the gate 23, so that itpermits passage of signals when the beam in the pick-up tube 4 generatessignals corresponding to the red light passed by the red line, filters.Thus, the signals passed by the clear strips initiate the sequentialkeying `of the gates 26, 2'?, and 28, so as to permit the l signals tobe passed separately through them 'in accordance with the colorrepresented by the particular signals. The operation of the gates isindicated by the vertical solid lines 35 between the black level l5 andthe waveform i4 of Figure 2.

It was stated above that the camera incorporating the principles of thisinvention is most useful in connection with a dot multiplex colortelevision system wherein the color information is sampled and thebrightness information is passed around the sampler. No sampler has beenshown in the drawings for this does not comprise a part of the camera.This is advantageous because the portion of the camera actually movedaround within the studio need only be comprised of the two pick-up tubes2 and 4, pre-amplilers for their signal outputs, and the optical systemcomprised of lens 6 and the half-silvered mirror 8. The rest of thecircuity, including the gates. filters and delay lines, can be locatedin the control room. Thus, the cables need only carry high voltage andscanning voltages to the cameras and only two output leads need returnfrom the cameras. The possibility of doing this is of extreme importancewhen the practical operation of studio equipment is taken into account,because the cameraman have all they can do to keep signals are applied,an amplitude selecting circuit the camera focused on the center ofinterest of the scene being televised. Thus, the more control that canbe placed in the control room the better.

It is to be emphasized yonce again that the y provisions of a singlecamera tube to pick up all the low-resolution chromaticity signals incombination with a pick-up tube producing the high detailed.`black-and-white information does not require a high degree ofregistration, and that the use of a line iilter with a relatively smallnumber of strips and with a clear strip between each series of green,red, and blue strips does not require a high degree of scanninglinearity or stability.

Having thus described my invention, what is claimed is:

A color television camera comprising in combination a first pick-up tubeand a second pick-up tube, Saidv pick-up tubes having means forDrojecting a beam of electrons and means for Icausing said electrons toscan a raster, optical apparatus for focusing an image to be televisedonto each of said pick-up tubes, means for causing the signals developedbythe scanning action of the beam in said second pick-up tube to have anamplitude determined by the intensity of successive sequences ofdifferent Acomponent colors, means for supplying a signal between atleast some of said sequences having an amplitude greater than any one ofthe signals in the adjacent sequence, a clamping circuit to which saidconnected to the output of said clamping circuit, an oscillator, meansfor controlling the frequency of said oscillator in response to theoutput of said amplitude selecting circuit, a plurality of gate circuitsadapted to receive said signals, a pulse forming circuit connected tothe output of said oscillator and delay lines connected between saidpulse forming circuit and each of said gates, the amount of delay beingsuch that the gates are successively conductive in synchronism with 'thechange in the component color represented by successive signals.

Great Britain V June 9, 1948'

